A/D converters can be used in many different applications to convert an analog signal into a digital signal. Since there is an analog portion to all A/D converters, that analog portion is often calibrated to make sure that the digital outputs are accurate.
CMOS image sensors may have a column parallel readout using an array of programmable gain amplifiers or PGAs. CMOS image sensors use A/D converters to produce the digital output indicative of the analog input. A typical column parallel image sensor may use successive approximation A/D converters. This requires that the A/D converters maintain a precise ratio in the ratio of Vref/2n, for example Vref, Vref/8 and Vref/32. Those references are sometimes generated on the image sensor chip by D/A converters. The digital control logic on the chip can then adjust those to correct for variations and adjust desired ratio due to mismatches and due to the formation process.